Effect of cold working on surface hardness

[metboss]

Hi All,

We had received SS 316L Rings (6” circular hollow ring) which will be used on Drilling Equipment as a retainer ring to hold packing elements. As per our client specification, all these Rings (100 No’s.) shall be hardness tested 100%.
However, from test records, it is observed that only 20% (20 No’s. on random sampling basis) has been hardness tested. When we contacted the supplier/manufacturer, they said that all these Rings are from the same heat treated lot and 20% testing would be suffice to justify the entire lot.
Moreover, all these Rings will be subjected to exterior skim cutting (2 mm machining) in our workshop.

My questions are:

Does “skim cutting or machining or any cold working after final heat treatment” affect hardness values ?
Does a light draw pass for size control impart some cold work to the outer surface which normally results in a slight increase in surface hardness ?
Does it require an additional heat treatment after skim cutting/machining/light draw pass to nullify the effect of cold working/strain hardening to control surface hardness?

Please advise.

 

[metengr]

Does “skim cutting or machining or any cold working after final heat treatment” affect hardness values ?

For machining, a light surface layer of cold worked material can form. For cold working, definitely.

Does a light draw pass for size control impart some cold work to the outer surface which normally results in a slight increase in surface hardness ?

Yes.

Does it require an additional heat treatment after skim cutting/machining/light draw pass to nullify the effect of cold working/strain hardening to control surface hardness?

It depends on your maximum hardness requirement by material specification or your own engineering specification maximum hardness for service.

 

[EdStainless]

Usually a little surface hardening isn't a bad thing.
It often goes along with the surface being slightly in compression which often helps performance.
I have seen a lot of 316L where the surface is slightly harder, how much depends on how you test it.
Most SS specs call out RB 90 max for annealed material.
If the layer of cold work is thin then RB with it's high load will hardly be effected at all.
It really comes down to why do you care?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear metboss,

Q1 - Ans: After exterior cutting, check hardness values at random (say 20% as per your vendor).

Q2 - Ans: Yes

Q3 - Ans: No

Regards.

DHURJATI SEN